In a basic 4WD vehicle, power goes from the transmission to a transfer case where it is divided between the front and rear axles.
In old school 4WDs, that’s all there was to it. It wasn’t a bad system, except for one big flaw. Because the transfer case sends equal power to every wheel, manoeuvring in a traditional 4WD could be difficult. For a car to make a turn properly, the wheel on the inside actually needs to turn slower than the outside wheel – if not, the inside wheel loses traction and spins, which is pretty inefficient.